43rd Lunar and Planetary Science Conference (2012)
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EVIDENCE FOR VOLCANISM IN AND NEAR THE CHAOTIC TERRAINS EAST OF VALLES
MARINERIS, MARS. Tanya N. Harrison, Malin Space Science Systems ([email protected]; P.O. Box 910148,
San Diego, CA 92191).
Introduction: Martian chaotic terrain was first described by [1] from Mariner 6 and 7 data as a “rough,
irregular complex of short ridges, knobs, and irregularly shaped troughs and depressions,” attributing this
morphology to subsidence and suggesting volcanism
as a possible cause. McCauley et al. [2], who were the
first to note the presence of large outflow channels that
appeared to originate from the chaotic terrains in Mariner 9 data, proposed localized geothermal melting
followed by catastrophic release as the formation
mechanism of chaotic terrain. Variants of this model
have subsequently been detailed by a number of authors [e.g. 3,4,5]. Meresse et al. [6] documented possible cinder cones, some with potential associated flows,
on the floor of Hydraotes Chaos. No other volcanic
features have previously been documented within or
around Hydraotes and the other chaos regions at the
eastern end of the Valles Marineris system.
Here I present observations of volcanic features associated with some of these chaotic regions, including
small shield volcanoes and extensive lava flows, as
imaged by the Mars Reconnaissance Orbiter (MRO)
Context Camera (CTX). At the time of writing, CTX
has acquired nearly complete coverage of Valles Marineris and its associated chaotic terrains (Fig. 1). I systematically inspected all CTX images along the entire
chasmata and chaos system to look for potential volcanic landforms such as cones, vents/shields, and lava
flows.
Observations and Interpretations:
Volcanoes. A cluster of domical features in the
southeastern portion of Hydraotes Chaos are visible in
CTX B19_017212_1809_XN_00N033W near 0.44°S,
33.18°W (Fig. 2a). This area is the lowest elevation
point in all of the chaotic terrains. These features exhibit fissures at their summits and appear to have associated flows, which bank up upon blocks on the chaos
floor. Some also display cones with summit craters on
their flanks, and a number of similar cones are visible
in the general vicinity. These domical features are
unique to Hydraotes compared to the rest of Valles
Marineris and the chaotic terrains, where no such features are observed. They are also morphologically distinct from the cones documented by [6]. Based on
morphology and the presence of associated flows, I
interpret the domical features to be small shield volcanoes.
Lava flows. Dark-toned flows appearing to have
originated from fractures around the margins of multi-
ple chaotic regions are visible in CTX images (Figs.
1,2). These fractures have widened since the formation
of the flows. The flows overtop and/or bank up upon
pre-existing topography such as crater ejecta blankets
(Fig. 2c). Flows are also observed originating from
fractures within some craters in the vicinity of the chaos regions. Potential lava flows are observed on a portion of the floor as Hydaspis Chaos, possibly associated with fissures on the chaos floor. As in Hydraotes,
these flows bank up against blocks on the chaos floor,
implying that if the flows are volcanic in origin, the
volcanism occurred after the formation of Hydaspis
Chaos.
The dark-toned material has retained more small
(sub-km) craters than the surrounding plateau material,
suggesting that it is more resistant to erosion [7]. This,
paired with the observation that the margins of the
flows are not significantly eroded, supports the interpretation that these are lava flows (as opposed to mudflows).
Cones/Mounds. Multiple small cones with summit
craters were identified in Hydraotes Chaos using CTX
(some of these were previously documented by [6]). In
some places, mounds lacking summit craters occur in
association with cone clusters. Similar cones were also
identified to the south in Aurorae Chaos, near the
mouth of Ganges Chasma, and in central and eastern
Coprates Chasma (some of the latter were also noted
by [4]). Cones with and without summit craters are
also observed to the north in Chryse Chaos. These may
potentially be rootless cones and/or cinder cones, as
suggested by previous authors where cones had been
identified [e.g. 4,6].
Discussion: None of the lava flows are observed to
have flowed from fractures into the topographic lows
of the chaotic terrains themselves, suggesting that volcanic activity associated with these fractures ceased
before the formation of the chaotic terrain as we see it
today. Evidence for volcanic activity after the formation of the chaotic terrains lies in the observations
of shield volcanoes, cones, and flows on the floor of
Hydraotes and cones, fissures, and flows on the floor
of Hydaspis. These features are unique to these chaos
regions however, suggesting that this latter period of
volcanic activity was much more limited in extent.
While these observations imply that volcanism occurred in the region before and after the formation of
the chaotic terrains, it does not directly hold any implications for the formation of the chaotic terrains. The
43rd Lunar and Planetary Science Conference (2012)
spatial distribution of the fractures and lava flows relative to the chaos may suggest that volcanism played a
role in chaos formation, as these features are not observed farther away from the chaos regions.
References: [1] Sharp R. P. et al. (1971) JGR, 76,
331–342. [2] McCauley J. F. et al. (1972) Icarus, 17,
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289–327. [3] Carr M. H. (1979) JGR, 84, 2995–3007.
[4] Lucchitta B. K. et al. (1994) JGR-Planets, 99,
3783–3798. [5] Rodriguez J. A. P. et al. (2005) Icarus,
175, 35–57. [6] Meresse S. et al. (2008) Icarus, 194,
487–500 [7] Edgett K. S. (2009) GSA, 41, 708.